Automatic transmissions include multiple clutches, which are mechanisms for transmitting rotation. These clutches are selectively engaged and disengaged to provide a desired gear ratio between the transmission input and output shafts. Advantageously, an automatic 6-speed transmission with a high first gear ratio, a second gear ratio at a considerable lower value from the first, and all gears other than first that are fairly close together, provides improved performance and fuel, economy. The launch performance is improved through increased torque at first gear, and the fuel economy is improved at all gears because the engine can work in a much more favorable operating region. However, this creates a challenge to the transmission's garage shift quality. An example of such a 6-speed transmission is the 62TE transmission, available from DaimlerChrysler Corp. of Auburn Hills, Mich.
Garage shifts include a shift from neutral to drive or reverse, a shift from drive to neutral, or a shift from drive to reverse or from reverse to drive. Traditionally, the D-N garage shift is an uncontrolled, ballistic vent shift. During a garage shift, all the torque will act on, as well as be released from, a powertrain mount system which typically causes annoying garage shift bumps. For example, during the D-N shift, the hydraulic fluid in a clutch, such as an Under Drive (UD) clutch, vents through a manual lever control value. As soon as the clutch loses its capacity, a turbine in a torque converter will go to its floating position because it loses its connection with an output driveshaft and all of the potential energy stored in the powertrain mount system will be released with the whole powertrain returning to neutral position in a fraction of a second, which creates the bump during the D-N garage shift. An N-D garage shift is a controlled shift. During the N-D shift, when hydraulic fluid fills a clutch (e.g., UD clutch), the clutch tries to connect the engine to the drive shaft, and as a result reaction torque will cause the powertrain case to wind to an opposite direction from the engine output shaft rotation. Finally, the powertrain ease will be balanced by the mount system. Since this process happens in a traction of a second, it will generate an N-D garage shift bump felt by vehicle occupants.
A typical way to improve the D-N garage shift quality is to slow down the vent rate of the clutch, such as in the UD clutch, by reducing the orifice size. However, this has a negative side effect on the vent rate during Rock Cycle when it usually needs a high vent rate, and the D-N shift may take too long during extremely cold conditions. A smaller orifice will also slow down the fast fill period delaying engagement during N-D shift. U.S. Pat. No. 6,558,293 to Skupinksi et al. discloses a garage shift control method which makes the N-D shift by initially shifting to an upper gear ratio followed by shifting to a low gear ratio when the shift to upper gear is substantially complete or when a predetermined period time has elapsed since shift initiation. The Skupinksi reference improves the N-D garage shift quality; however the vehicle occupants may feel the down shift (because of the torque change from upper gear to lower gear ratios, especially for aggressive drivers, who launch the automobile as soon as putting the shift lever to the drive position) from upper gear to the first gear ratio. Further, the Skupinksi reference does not address the D-N shift, focusing solely on the N-D shift.
Methods and systems are needed to improve both D-N and N-D garage shift quality which overcome the aforementioned limitations without compromising on vehicle performance.